This invention relates to an electronic controlled fluid suspension system for controlling a vehicle attitude.
In order to improve ride comfort, maneuverability and stability of a vehicle, various types of systems for reducing pitch and roll have been developed. One of them is presented in the publication of unexamined Japanese Utility Model Application No. 61-59414. In this prior art reference, when a steering wheel is suddenly turned at a steering angle speed exceeding a preset value, an advance control is executed. Namely, fluid is rapidly supplied to or discharged from a fluid actuator during a controlled time interval which is obtained from a map showing a relation between the controlling time interval and a vehicle speed and a steering angle speed. On the other hand, when the wheel is slowly steered at a steering angle speed smaller than the predetermined value, a feedback control is executed. Namely, fluid is supplied to or discharged from the fluid actuator during a controlled time interval based on a predetermined map relating the controlled time interval and a vehicle height which is maintained for a predetermined time interval.
The above-mentioned prior art suspension system, however, includes some problems. The advance control and the feedback control are executed alternatively on the basis of a predetermined steering angle speed. In such case, it is practically impossible to equalize the control amount of the advance control and that of the feedback control when the control mode is switched from one to the other, because it is difficult to take into account the friction coefficient and irregularities of a road surface and a load of the vehicle immediately after the control mode is switched. The difference of the control amount between the above-mentioned two modes is apt to produce a momentary change in the vehicle height, which impedes smooth control of the vehicle attitude.
There is also another problem. When the steering wheel is slowly turned, the feedback control is executed after the vehicle height is maintained for a predetermined time interval. Since the feedback control is not carried out until the predetermined time interval ends, smooth attitude control cannot be executed during the above-mentioned time interval.